There have been several primers and processes developed over the years to improve the adhesion of materials to polypropylene and other polyolefins. Polyolefins are generally inert, paraffin-like polymeric substrates which are non-polar and very difficult to paint, coat or bond. They are further characterized as having low critical surface tensions of wetting, e.g., 35 dynes/cm or less which make them difficult to wet with paints, coatings and adhesives. Some of the processes developed to date to enhance the adhesion to polyolefins are flame treatment, corona discharge treatment, photo sensitized ultraviolet irradition, gas plasma treatment, acid etching, electron beam irradition and perhaps others. These processes generally enhance adhesion by either increasing wetting via oxidation (increase the critical surface tension of wetting), by cross-linking to increase surface mechanical strength, by increase in surface porosity to increase mechanical bonding by diffusion, by general surface cleaning or by combinations of these effects.
Primers have been developed for increasing adhesion to polyolefins and are generally non-polar or polar/non-polar in nature to help provide a "bridge" between the substrate (the polyolefin) and the adhesive, coating or paint. Paints, coatings and adhesives are not necessarily always polar, but generally are, and the polar/non-polar primer has both an affinity for the non-polar substrate and the polar paint, coating or adhesive.
The patents of Bragole U.S. Pat. Nos. 3,600,389; 3,607,536; 3,619,246; 3,627,609; 3,892,885; 4,321,307; and 4,764,370 teach various manifestations of photosensitized irradiation techniques to enhance bonding to polyolefin substrates. Such manifestations include the use of isocyanates or silanes or magnesium reacted phenolic resins with photosensitized ultraviolet irradiation. Other manifestations include the presence of isocyanate at the time of photosensitized ultraviolet irradiation while others use photosensitized irradition in combination with reactive products such as those containing isocyanates, acrylic monomers or epoxies. Some of the current techniques, while effective for some applications, produce surface changes which are not permanent. Others suffer from the fact that large quantities of hazardous waste have to be removed which is very expensive. Others suffer from universality and some are too expensive to have had widespread use.
The primers appear to be more effective with paints and coatings than with adhesives. While the primer adheres well to the polyolefin it frequently suffers from inadequate adhesion to the adhesive.
I have now found that irradiation of these primers not only improves their subsequent adhesion to adhesives but also improves the adhesion of the primer to the substrate, e.g. the polyolefin surface.
It is believed the primer becomes engrafted to the polyolefin substrate surface and cross-links during continual exposure to irradiation. That is, it has been found that multiple exposure to relatively low radiation produces superior bonding results when compared to single or a few exposures to relatively high radiation, the relatively high radiation being substantially the value of the total amount of low radiation exposure. The net effect is (1) a stronger union of the primer to the polyolefin surface than is possible without irradiation (2) an insolubilizing of the primer making it more resistant to heat and chemical attack and (3) the previously disclosed improved adhesion of the engrafted primer to adhesives. While the alteration of the primer can accommodate and interact with the active ingredients (curative) of the two-part adhesives, I have found that the irradiated primer surface is active enough to accommodate one-part adhesives as well, such as one-part polyurethanes and neoprenes adhesives. In addition to chlorinated polypropylene primers, others I have found that other primer types such as varying molecular weight terpolymers of vinyl chloride, vinyl acetate, and maleic acid are also useful as primers, after irradiation, alone or in admixture with the chlorinated polypropylene primers.
It appears that primers with certain characteristics are required to be effective in the primer irradiation process and these characteristics are at least the struoture and molecular weight of the primer. For example, the percent halogen; i.e. chlorine, should range between 5 to 50% by weight and have a number dosage molecular weight of about 5,000 to 100,000. Further, it appears the higher the molecular weight, the more effective the primer is. Lastly, exposure to successive low dosages of radiation rather than a single or few doses at a level which would be equal to the columlative exposure of the relative low dosages is a further characteristic. The presence of some hydrogen on carbon atoms adjacent to carbon atoms bearing a halogen(s) also appear to be important factors. It is believed that the irradiated primer system will work on all substrates, including polyolefins, which have at least one hydrogen atom in a repeating unit of structure within the polymer.